Linear polyethylenes having an average molecular weight of more than 500,000 g/mol, particularly at least 2.5.times.10.sup.6 g/mol to 10.sup.7 g/mol, are called ultra-high molecular weight polyethylenes (UHMW-PE). The molecular weights are understood as meaning the values determined by viscometry and a method for their measurement is described, for example, in CZ-Chemietechnik, Vol 4 (1974), page 129 et seq. Various processes are known for the preparation of UHMW-PE and a proven process which operates under low pressure using mixed catalysts of Ti(III) halides and organo-aluminum compounds is described in DE-B No. 2,361,508.
UHMW-PE is distinguished by a number of industrially important properties, namely high impact strength which still shows good values even at low temperatures should be emphasized. Even the severest exposure to impact does not lead to fracture. It is resistant to stress corrosion and is also dimensionally stable and tear-resistant at elevated temperatures. The high abrasion resistance of UHMW-PE, its low coefficient of friction compared with other materials and its excellent resistance to aggressive media furthermore are remarkable.
On the basis of this profile of properties, UHMW-PE has found acceptance as a special material in many branches of industry such as the use of UHMW-PE for the production of weaving loom pickers and similar components in the textile industry which are subjected to high stresses, as a material for rollers, valves, slides, strippers and guide strips in machine construction, and for lining bunkers, coke chutes and conveyor troughs in mining. It is employed in filtration for manufacture of filter elements, in the packaging industry for the production of conveyor stars and guide and deflection rollers for bottling machines, and in electrical engineering for production of insulating components in the high and ultra-high frequency sector.
In contrast to low molecular weight grades of polyethylene which are mainly supplied in the form of granules by producers, UHMW-PE is sold as powder which is unfavorable for transportation, distribution, storage and processing. Ultra-high molecular weight polyethylene is supplied as a powder due to its thermal properties. On heating, UHMW-PE does not melt, but merely passes into a viscoelastic state in which the individual polymer particles soften but still retain their shape even at 200.degree. C. They combine with one another to give larger particles which are only loosely joined together and which easily fall apart again.